diff --git a/include/osmocom/msc/transaction.h b/include/osmocom/msc/transaction.h
index 0760c07..e82f0a5 100644
--- a/include/osmocom/msc/transaction.h
+++ b/include/osmocom/msc/transaction.h
@@ -98,11 +98,11 @@
 
 
 
-struct gsm_trans *trans_find_by_id(struct ran_conn *conn,
+struct gsm_trans *trans_find_by_id(const struct ran_conn *conn,
 				   uint8_t proto, uint8_t trans_id);
-struct gsm_trans *trans_find_by_callref(struct gsm_network *net,
+struct gsm_trans *trans_find_by_callref(const struct gsm_network *net,
 					uint32_t callref);
-struct gsm_trans *trans_find_by_sm_rp_mr(struct ran_conn *conn,
+struct gsm_trans *trans_find_by_sm_rp_mr(const struct ran_conn *conn,
 					 uint8_t sm_rp_mr);
 
 struct gsm_trans *trans_alloc(struct gsm_network *net,
@@ -111,9 +111,9 @@
 			      uint32_t callref);
 void trans_free(struct gsm_trans *trans);
 
-int trans_assign_trans_id(struct gsm_network *net, struct vlr_subscr *vsub,
+int trans_assign_trans_id(const struct gsm_network *net, const struct vlr_subscr *vsub,
 			  uint8_t protocol, uint8_t ti_flag);
 struct gsm_trans *trans_has_conn(const struct ran_conn *conn);
-void trans_conn_closed(struct ran_conn *conn);
+void trans_conn_closed(const struct ran_conn *conn);
 
 #endif